This invention relates to the combustion of pulverized coal in particular for slowly reacting coal like anthracite, lean coal etc. with a volumetric heat release rate corresponding to the flame volume of more than 2 million Kcal/m.sup.3.h.at.
Different types of designs for coal-dust firing have been known for a long time. They are divided into those with dry ash removal and those with liquid ash removal. Dry ash removal is usually employed with coal high in inerts, especially brown coal, in order to profit by the heat content of the ash in the boiler flue passes.
Dry ash removal requires sufficiently low furnace temperatures. These are usually achieved by the corresponding volumetric heat release rate. In this case there are volumetric heat release rates of 100,000 to 200,000 Kcal/m.sup.3.h.at. This again means an accordingly large volume of the radiation chamber and, accordingly, a large boiler volume. The low furnace temperatures lead to partially poor combustion. This again has led to more or less complicated methods for recycling unburnt ashes.
High volumetric heat release rates in boilers fired by pulverized coal can only be achieved with so-called "wet bottom" boilers up to now. In this case there are volumetric heat release rates of the order of 1 million Kcal/m.sup.3.h.at and more when operating with liquid slag. In certain areas of the combustion chamber they consciously put up with the fact that part of the heating surface is covered with more or less molten slag. No doubt this permits the design of accordingly smaller boilers because of the higher volumetric heat release rates and of the smaller combustion chamber. Because of the necessity of maintaining high temperatures however, wet bottom boilers are not fit for the firing of small amounts of pulverized coal. In particular, wet bottom boilers do not permit the ON/OFF control usually preferred in central heating control technology as it would result in the solidification of the slag flow at every boiler stop.